In the sulphur circulation of a kraft pulp mill (sulphate pulp mill), non-condensible gases are produced which contain considerable amounts of sulphur compounds, such as hydrogen sulphide, methylmercaptan, dimethyl sulphide, and dimethyl disulphide. To eliminate odour problems, these so-called malodorous gases are incinerated, for example, in a separate incineration plant for non-condensible gases, wherein carbon dioxide, water and sulphur dioxide are produced. To minimize sulphur emissions caused by flue gases from the incineration plant, sulphur dioxide is normally bound with a sodium hydroxide solution in a caustic scrubber downstream of a waste heat boiler. By exploitation of the resulting chemicals (Na2SO3 and NaHSO3) in different sections of the kraft pulp mill it is possible to control the Na/S balance of the process. By this technique it is possible to utilize a part of the chemicals.
Most commonly, however, the Na/S ratio is still adjusted by dissolving fly ash from the soda recovery boiler into water and by discharging it as sewage from the process. The need for dissolution may be several tens of tons of Na2SO4 per day. Furthermore, the washing, extraction, and dissolution/crystallization of fly ash have become more common as the need for removing harmful substances increases. In the above-mentioned processes, a considerable amount of sodium is always washed out into the water system and must be replaced by supplying, for example, caustic into the chemical recirculation. The cost of the caustic makeup at the plant may amount to even 1 million euro per year.
Attempts have been made to replace sodium hydroxide in gas scrubbing because of the entailed costs of chemicals. For this reason, Finnish patent 104504 presents the use of an alkaline calcium compound for the scrubbing of flue gases containing sulphur dioxide produced during the incineration of non-condensible gases in a kraft pulp mill. In said patent, the calcium compound is common limestone or another alternative calcium compound, such as alkaline calcium compounds of ash, particularly fly ash, from a bark-fired boiler in the same kraft pulp mill. The patent discloses how the calcium compound (calcium sulphate and sulphite) produced in a reaction with sulphur dioxide is finally led into the sewage system of the kraft pulp mill.
Furthermore, Finnish patent 93869 discloses a so-called dual alkali process in which a solution containing sodium bisulphite from a caustic scrubber of flue gases emitted by the incineration of non-condensible gases is treated with waste calcium oxide obtained from lime mud reburning. This is, in principle, a caustic scrubber in which sodium bisulphite is regenerated with calcium oxide to form sodium hydroxide which can be utilized in the caustic scrubber. At the same time, calcium sulphite and sulphate are produced which can be stored as waste. The method contains several steps and is difficult to implement.
FIG. 3 of Finnish patent 103532 shows a method in which calcium oxide obtained from the lime reburning kiln of a pulp mill is mixed into water and the calcium hydroxide thus obtained is led into a gas scrubber for scrubbing flue gases from a power boiler of the pulp mill, the flue gases containing sulphur compounds because the non-condensible gases of the pulp mill are also incinerated in the power boiler. This method is based on the utilization of calcium oxide and its conversion into calcium hydroxide.
Until now, there has been no method for processing flue gases produced by the incineration of non-condensible gases at the kraft pulp mill in a simple plant where the Na/S balance and the harmful substances of the process could be simultaneously controlled.